Hip abduction can prevent posterior edge loading of hip replacements.
Richard J. van Arkel1, Luca Modenese2, Andrew T.M. Phillips2, Jonathan R.T. Jeffers1
1
Medical Engineering, Department of Mechanical Engineering, Imperial College London,
London
2
Structural Biomechanics, Department of Civil and Environmental Engineering, Imperial
College London, London
Proofs and reprint requests:
Jonathan Jeffers
Department of Mechanical Engineering, Imperial College London, London, SW7 2AZ,
United Kingdom
j.jeffers@imperial.ac.uk
Telephone: +44 (0)20 7594 5471
Fax: +44 (0)20 7594 5702
Running Title: Edge loading in hip replacements
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ABSTRACT
Edge loading causes clinical problems for hard-on-hard bearings and edge loading wear scars
are present on the majority of retrieved components. This study asks the question: are the
lines of action of hip joint muscles such that edge loading can occur in a well-designed, wellpositioned acetabular cup. A musculoskeletal model, based on cadaveric lower limb
geometry, was used to calculate for each muscle, in every position within the complete range
of motion, whether its contraction would safely pull the femoral head into the cup or
contribute to edge loading. The results show that all the muscles that insert into the distal
femur, patella or tibia could cause edge loading of a well-positioned cup in deep hip flexion.
Patients frequently use distally inserting muscles for movements requiring deep hip flexion,
such as sit-to-stand. . Importantly, the results, which are supported by in vivo data and
clinical findings, also show that risk of edge loading is dramatically reduced by combining
deep hip flexion with abduction. Patients, including those with sub-optimally positioned cups,
may be able to reduce the prevalence of edge loading by rising from chairs with the hip
abducted.
Key words: avoid edge loading, muscles, hip, ceramic-on-ceramic, metal-on-metal
1.
INTRODUCTION
Edge loading causes clinical problems for hard-on-hard hip replacements: for metal-on-metal
(MoM) implants edge loading wear can lead to pseudotumours and early revision1 and for
ceramic-on-ceramic (CoC) bearings, edge loading has been related to higher wear rates and
squeaking hips.2
Edge loading is a term used to describe the increased contact stress resulting from a decreased
contact area between the acetabular cup and femoral head at the rim of the cup. It occurs
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when the cup provides insufficient coverage of the head preventing a full circular contact area
from developing around the hip joint contact force vector.3 Research has shown that this
mechanism particularly affects MoM implants with reduced cup subtended angles,3 and/or
poor cup positioning,4 as these factors bring the rim of the cup closer to the path of the contact
vector and expose the hip to edge loading.5 However, there is also clinical evidence of
unexplained edge loading wear on retrievals from well-designed, well-positioned MoM
components3 and a recent in vivo MoM resurfacing study showed that posterior edge loading
occurs in all hips (30/30) in all patients (19/19) when extending from deep hip flexion when
rising from a chair.5
Edge loading can also occur as a consequence of near-dislocation events: anterior
impingement in deep hip flexion and internal rotation, the most common mechanism, causes
small subluxations of the femoral head which exposes it to posterior edge loading on the hard
edge of acetabular cup leading to extreme contact stresses and wear.6 However CoC implant
retrievals have shown that posterior edge loading wear scars are present on the majority of
bearings and occur most commonly in the absence of impingement.7,8
Given the high incidence of posterior edge loading reported in the absence of impingement,
and the strong influence of muscles on the hip joint contact force,9 we hypothesized that the
line of action of muscles are such that edge loading can occur in all hips when they are deeply
flexed during routine activities, and so this study addresses the following questions:
-
Are the lines of action of hip joint muscles such that they could cause edge loading of
a well-designed, well-positioned acetabular cup?
-
How sensitive are the results to geometrical variation of the acetabular cup through
changes to the implant design or orientation?
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2.
2.1
METHODS
Muscle Contribution to Edge Loading
A lower limb musculoskeletal model was developed based on a digitised right leg cadaveric
specimen which detailed muscle origin and insertion points.10 More detailed information
about the model can be found in a previous study which compared computed hip joint contact
force magnitudes with those measured in vivo.11 The model’s accurate muscle geometry
included an anatomical wrap for the iliopsoas muscle fibres around the pelvis to ensure it
pulled the femur in the correct direction. To keep representative muscle geometry throughout
a complete range of motion, additional muscle wrapping surfaces were applied to the gluteus
maximus superior fibres, gluteus maximus inferior fibres, the gemelli and obturator internus,
as detailed in Appendix A.
The full available range of motion of the hip for an adult male12 was discretised into 5°
positions of hip flexion (-10° to 120°), abduction (-25° to 40°) and rotation (-40° to 40°)
giving a total of 6426 different hip orientations. Angles were referenced in accordance with
the ISB recommendations for joint coordinate systems.13 OpenSim version 2.4.014 was used to
place the musculoskeletal model in each of these static positions and the direction of the force
vector exerted by each muscle onto the pelvis was calculated using an OpenSim plugin, which
is available for free download together with detailed documentation.15 An overview of how
the plugin works is also included in Appendix B.
The hip was modelled in MatLab (version 2011b, The MathWorks Inc., Austin, USA) as a
Ø28mm bearing with a subtended angle of 168° (representative of a Biolox Forte cup)
implanted at 20° operative16 anteversion and 45° inclination. A conservative edge loading
risk-zone, which allows for the circular contact patch surrounding the force vector, was
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defined within 5° of the cup edge. For each muscle, it was calculated if its contraction would
safely pull the femoral head into the cup, Figure 1A, or contribute towards creating an edge
loading force vector, Figure 1B.
2.2
Effects of Implant Design
The muscle contribution to edge loading was then re-calculated for a well-positioned cup for
all the implant designs listed in Table 1. The effect of varying the edge load risk-zone was
studied by varying the angle in the range 5-30°, which is representative of the range of
possible contact patch semi-angles.5,17,18
2.3
Effects of Implant Orientation
The effects of different acetabular orientations were investigated using the original cup design
and by varying the operative implantation angles for all nine possible combinations of 5°, 20°
and 35° anteversion with 30°, 45° and 60° inclination.
3.
3.1
RESULTS
Muscle Contribution to Edge Loading
All the muscles that inserted into the distal femur, patella or tibia can contribute to edge
loading of a well-positioned cup within a normal range of motion, whereas other large
muscles, such as the gluteus medius, cannot. Figure 2 lists the muscles included in the study
and the percentage of positions in the complete range of motion where the line of action of
that muscle contributed to an edge loading hip contact force.
The risk of edge loading was particularly prevalent during deep hip flexion. This is shown in
Figure 3 for a well-positioned cup; the percentage of muscles that could contribute to edge
loading increased from 0% to 39% (9/23) as hip flexion increased from 80° to 100° with
neutral abduction and rotation.
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Hip abduction dramatically reduced the possible muscular contribution to edge loading in
deep hip flexion (Figure 3); at an abduction angle of 20° or greater no muscles contributed to
edge loading up to 95° of flexion. Hip flexion with adduction had the opposite effect; when
the hip was in 20° adduction, muscles started causing edge loading above 50° flexion. Internal
or external rotation of the hip made little difference to the risk of edge loading.
3.2
Effects of Implant Design
Decreasing the subtended angle of the acetabular cup arc both increased the maximum
possible muscle contribution to edge loading, and decreased the angle of hip flexion at which
muscle contribution to edge loading was possible (Figure 4). However, changing the size of
the bearing in isolation did not affect the possible muscular contribution to edge loading.
Changing the edge load risk-zone had the same effect as decreasing the subtended angle as
these changes both reduced the safe coverage of the femoral head. For example, two bearings
with subtended angles of 168° and 152° and edge load risk-zones of 13° and 5° respectively
are equivalent (their safe coverage arcs are 142°).
3.3
Effects of Implant Orientation
For all cup positions investigated the general trend was the same as shown in Figure 3, the
percentage of muscles that can contribute to edge loading increased rapidly at a given angle of
hip flexion, was highest in deep flexion, and abducting the hip had a protective function.
Internal and external rotation had a larger effect in some cup positions in comparison to a
well-positioned cup; however the dominant effect was still driven by hip flexion, then
ab/adduction. The following trends are based on data from the complete range of motion;
however many of them can be seen in the data presented in Figure 5.
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Low anteversion decreased the angle of flexion at which edge loading could occur but had
little effect on the maximum number of muscles that could edge load a hip joint; it effectively
shifted the lines shown in Figure 3 to the left. High anteversion had the exact opposite effect;
it allowed higher angles of hip flexion before large numbers of distal muscles could contribute
to edge loading.
Low inclination had two effects: firstly it increased the maximum number of muscles that can
cause edge loading forces over all angles of hip flexion. This was because some of the short
external rotators and obturator muscles had contact vectors that were in the inferior portion of
the risk zone. Secondly, it reduced, but did not eliminate, the effect of abducting the hip in
deep flexion on the number of muscles that can contribute edge loading force components.
High inclination had three effects: firstly, it decreased the number of distally inserting
muscles that can contribute to edge loading in flexion. Secondly, it increased the effect of
abducting the hip during flexion. Thirdly it opened up the possibility of the iliopsoas muscles
being able to contribute to edge loading forces during low hip flexion or extension angles, and
also the distally inserting muscles when the hip was adducted in low flexion or extension.
Combining high/low anteversion with high/low inclination provided a combination of the
above effects. For example low inclination and low anteversion resulted in very high muscle
contribution to edge loading at lower angles of hip flexion.
3.4
Comparison with Existing Data
Bergmann’s in vivo data9 provides kinematic and force data for sixteen trials of sit-to-stand. It
was used to test the correlation between the angle of flexion, abduction and rotation of the hip
at the point of maximum hip joint contact force (which occurs at, or shortly after the point of
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seat off and maximum flexion), and two angles that define how much the force points into the
acetabular cup: α in the transverse plane, and β in the sagittal plane (Figure 6).
At maximum load, there were strong, significant correlations between the angle of hip joint
abduction and α (Figure 6a, r=0.85, p-value<0.001), and the angle of hip flexion and β
(Figure 6b, r=-0.91, p-value<0.001). For the purposes of studying the maximum load in deep
hip flexion at seat off, the trial HSRCU3 is not representative; this trial has unique dynamics
and the HIP98 data shows that the maximum load occurs much later than the point of seat off.
The maximum load in this trial is not a good measure to study the force at seat off and when
this trial is excluded a stronger correlation is observed (Figure 6a, r=0.94, p-value<0.001).
4.
DISCUSSION
This study has shown that the lines of action of distally inserting muscles can contribute to
edge loading of a well-positioned acetabular cup when the hip is in deep flexion and
abducting the flexed hip moves the lines of action of these muscles away from the edge and
into a safe-zone inside the acetabular cup. This is because the lines of action of the distally
inserting muscles are tied to movements of the femur (Figure 1). The positive benefit of hip
abduction has been shown to be true for all the acetabular cup designs (Table 1) and
orientations tested (Figure 5). Incorporating hip abduction activities in the deep flexion range
of motion, like sit-to-stand, may be a useful rehabilitation exercise for patients to help avoid
edge loading wear, and this may be particularly beneficial for patients who have been
implanted with the ASR implant. Moreover, combining high angles of hip flexion with
abduction could also help prevent shear dislocation (without impingement)19,20 by bringing
the lines of action of all the muscles to within the acetabular cup. Abduction of a flexed hip
also moves the femoral neck and surrounding bone away from the anterior portion of the
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acetabulum and pelvis, the most common deep flexion impingement site,20,21 which adds
further weight to the finding that hip abduction in deep hip flexion is of benefit to patients.
The adopted methodology is purely geometrical and does not include explicit calculation of
the hip contact force vector because the individual muscle contributions are assessed only
with respect to their direction. Despite this limitation, the data shows strong correlation to the
resultant load vector measured in telemeterised implants.
It was shown in a recent
investigation that a model based on the same anatomical dataset adopted here could
potentially reproduce the hip contact force direction measured in vivo, but the optimisation
techniques currently employed for estimating the muscle forces are unable to yield muscle
recruitment adequate to provide accurate estimations of that vector.22 Therefore, by only
considering the direction of each muscle’s force vector, some of the limitations associated
with determining the magnitude of the compound joint reaction force through musculoskeletal
modelling are avoided. By not calculating the compound force vector, this study cannot
comment on the exact angles of flexion and abduction where edge loading occurs. However,
this study is able to show that the lines of action of the hip muscles are such that edge loading
of a well-positioned cup in the absence of subluxation is only possible in deep flexion, and
abducting the hip prevents this.
To verify the conclusions drawn, in vivo force data9 was tested for the trends found by this
study and Figure 6 shows that measurements from instrumented implants corroborate with the
findings: during sit-to-stand activity, posterior edge loading becomes possible as the direction
of the compound hip joint force relative to the pelvis is highly correlated with the position of
the femur, and hence in deep hip flexion the force vector points more posteriorly. Importantly,
the in vivo data also supports the result that activity modification can reduce the risk of edge
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loading: higher hip abduction at the point of seat off was strongly correlated with a more
medially angled force relative to the pelvis, and hence a force that points more inbound and is
further away from the posterior edge of the acetabulum (Figure 6a).
Rising from a chair requires in excess of 100° hip flexion.19 This movement relies on
considerable muscle force from the distally inserting hamstrings, rectus femoris and gluteus
maximus, and little contribution from the gluteus medius and short external rotators.20,23,24
These are the muscles we have found can contribute to creating an edge loading force (Figure
2) during deep hip flexion (Figure 3) are known to be highly active during sit-to-stand, whilst
muscles that we show provide a protective function are not.
The implant sensitivity study showed that decreasing the subtended angle of the acetabular
cup increased the possibility of muscle contribution to edge loading (Figure 4). This supports
results from explanted MoM bearings which show that cups with reduced subtended angles
edge loaded significantly more, and suffered significantly higher wear rates.3 For new implant
designs with reduced subtended arc angles, it is important to calculate the expected size of the
contact area, which can vary from 15° for a CoC bearing,17 to 16-30° for a MoM bearing.5,18
This is because the contact area size determines the edge load risk-zone and it is the
combination of a low subtended angles and large contact areas leads to excessive edge
loading wear and poor clinical results.3
The results of this study also support findings from ceramic retrievals where the majority of
edge loading wear was shown to occur posteriorly during high hip flexion7,8 with low cup
anteversion increasing the risk.25 Interestingly, this study shows that high inclination can help
protect against posterior edge loading by moving the inferior edge of the cup more laterally
and thus in an anteverted cup provides more posterior coverage of the femoral head.
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However, high inclination should be avoided as it can expose the joint to superior edge
loading during gait, which has severe consequences.4,8 Indeed recent MoM resurfacing
research using anterior-posterior X-rays suggest that low inclination is beneficial, particularly
for small bearing sizes.26 However, Figure 5 shows that low inclination can increase the risk
of posterior edge loading and so if used it should be combined with higher anteversion to
provide better coverage of the femoral head throughout the range of motion.
Figure 2 shows that the muscles damaged in the two most common surgical approaches
(lateral: gluteus medius and minimus, posterior: short external rotators)27 never cause edge
loading of well-positioned cups. Intraoperative repair and strengthening of these muscles
during rehabilitation may provide further means to reduce the risk of edge loading as
weakened muscles may lead to the patient substituting their function for a distally inserting
alternative28 which could contribute to an edge loading force.
Research has shown that edge loading can be caused by soft tissue laxity leading to
microseparation during gait,29 by impingement in deep hip flexion with internal rotation,6 or
by low subtended angles and/or high inclination resulting insufficient superior coverage of the
femoral head.4,5 This study does not discount these phenomena but provides an additional
mechanism by which edge loading could occur in the absence of mal positioning or
subluxation through soft tissue laxity or impingement.
In answer to the research questions posed, this study shows that all the distally inserting
muscles could cause edge loading of well designed, well-positioned acetabular cups when the
hip is deeply flexed. Low subtended arc angles, and suboptimal cup orientation can increase
the risk of edge loading through muscle action, but does not alter the general trend observed
for a well-designed, well-positioned cup. However, the most important finding of this study is
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that all patients, regardless of how their prosthesis was designed or implanted, can reduce the
prevalence of posterior edge loading, and perhaps dislocation, by introducing hip abduction to
activities that require deep hip flexion; this can easily be implemented for activities of daily
living such as rising from a chair and stooping by separating the knees before performing the
movement.
5.
ACKNOWLEDGEMENTS
This study was funded, in part, by the Engineering and Physical Sciences Research Council
and the Institution of Mechanical Engineers. There are no conflicts of interest.
6.
REFERENCES
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15. Modenese, L, Phillips, ATM, Thibon, A. 2012. OpenSim plugin to extract the muscle
lines of action. Accessed: 16 Jan 2013 from https://simtk.org/home/force_direction/
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16. Murray, DW. 1993. The definition and measurement of acetabular orientation. J. Bone
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Table 1
The models and dimensions of implant designs studied
Material Couple
Implant
Head diameter (mm)
Subtended Angle (°)
CoC
Biolox Forte
28
168
CoC
Delta Motion
36
168
MoM
Adept
38
161
MoM
Adept
58
161
MoM
ASR
59
152
MoM
ASR
39
144
Page | 15
Figure 1 (A-B) Diagrams of the line of action of the rectus femoris (blue line) and the
reaction forces at the hip joint (red/green arrows) at 90° flexion and neutral rotation. The
acetabular cup liner is divided into two parts, a green safe zone and a red edge load risk-zone.
(A) The hip is abducted 20° and the rectus femoris pulls the femoral head (blue sphere) into
the cup safe zone. (B) The hip is adducted 20°, now the line of action of the rectus femoris
pulls the head out of the cup and thus it could contribute to an edge loading contact vector.
Representative images from the musculoskeletal model are shown.
Page | 16
Figure 2 List of the muscles included in the study indicating the percentage of positions in
the complete range of motion at which each muscle could contribute to an edge loading force
vector in a well-positioned cup.
Page | 17
Figure 3 The percentage of muscles that can contribute to edge loading as a function of hip
flexion with neutral rotation and different ab/adduction in a well-positioned cup.
Figure 4 The effect of reducing the subtended angle of the cup arc on the possible muscle
contribution to edge loading for a well-positioned cup with neutral hip abduction and rotation.
Page | 18
Figure 5 The number of muscles that can contribute to edge loading at 100° hip flexion and
neutral hip rotation with varying hip abduction and acetabular orientation.
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Figure 6 The correlation the direction of the contact vector relative to the pelvis and the
position of the hip. Points are labelled with the first letter of the trial name and trial number
according to the sit-to-stand trial in HIP98 (e.g. H1 = HSRCU1 in HIP98) and lines of best fit
are shown. The trial HSRCU3 (H3) has abnormal dynamics and is highlight in red. A 5° varus
angle at the knee, which does not affect the correlation statistics, was used to convert from
Bergmann’s z-axis9 to the ISB’s y-axis.13 Images that define α and βare shown, the compound
reaction force at the pelvis is the green arrow, and the femoral axis is the blue dashed line.
Page | 20